1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus of an internal combustion engine, which has a particulate filter and an EGR device.
2. Description of Related Art
To address environmental concerns, it has been demanded to reduce the particulates, i.e., particulate matter (hereinafter, simply referred to as PM) and nitrogen oxides (NOx), which are contained in the exhaust gas of a diesel engine. The PM can be removed from the exhaust gas by providing a particulate filter (hereinafter, referred to as DPF) in an exhaust gas passage to collect the PM. The collected PM of the DPF is periodically combusted to recover a PM collecting performance of the DPF. In the DPF, a large number of passages, each of which is defined by porous partition walls, is formed. When the exhaust gas passes through the porous walls of the DPF, the PM contained in the exhaust gas is captured by the porous walls. The regeneration of the DPF is performed when the accumulated amount of the PM exceeds a predetermined amount.
The regeneration of the DPF is performed when the accumulated amount of the PM exceeds a predetermined amount. In the regeneration of the DPF, the hot (high temperature) exhaust gas is supplied to the DPF to increase the temperature of the PM equal to or higher than the combustion temperature of the PM. For example, substantial retardation of the main fuel injection timing or implementation of a single post fuel injection or multiple post fuel injections after the main fuel injection may be used as a temperature increasing means at the time of regenerating the DPF. By selecting the suitable temperature increasing means, which corresponds to the current operational state, the temperature of the DPF can be effectively increased to a target temperature (e.g., 500 to 650 degrees Celsius).
Furthermore, NOx emissions can be reduced by providing an exhaust gas recirculation (EGR) device. The exhaust gas after the combustion contains a large amount of inactive gas, such as water vapor or carbon dioxide. When a portion of the exhaust gas is recirculated into the intake air passage as the EGR gas, the combustion temperature is reduced to limit generation of NOx. In this instance, an EGR cooler may be provided in an EGR passage, which connects between the exhaust gas passage and the intake air passage, to cool the EGR gas. By cooling the EGR gas through the EGR cooler, a charging efficiency of the EGR gas is increased to slow down the combustion and thereby to increase the above effect. A bypass passage may be arranged in parallel to the EGR passage, in which the EGR cooler is provided. At the time of engine start, through use of the bypass passage, the EGR gas may be returned to the intake air passage without passing through the EGR cooler. This technique is disclosed in, for example, Unexamined Japanese Patent Publication Number JP2003-51791A and European Patent Publication Number EP0987427B1.
Here, it has been proposed to use the EGR as a means for improving the temperature increasing performance at the time of DPF regeneration. When the EGR is performed, the hot (high temperature) exhaust gas is introduced from the EGR passage to the intake air passage. Thus, the temperature of the intake air supplied to the combustion chambers of the cylinders is increased. As a result, the temperature of the exhaust gas supplied to the DPF is advantageously increased while the NOx is advantageously reduced.
However, in the case of the previously proposed temperature control system for controlling the temperature during the DPF regeneration, the temperature of the exhaust gas near an exhaust manifold immediately after the outputs of the combustion chambers would possibly be increased to 700 degrees Celsius or higher when the temperature of the exhaust gas on the upstream side of the DPF or on the downstream side of the DPF is adjusted to the target temperature. In the EGR valve, which controls the EGR flow rate, a flow passage forming member for conducting the exhaust gas is often made of a material that uses aluminum as its base material. Thus, when the hot exhaust gas is introduced into the flow passage forming member of the EGR valve, corrosion would possible take place in the flow passage forming member to deteriorate durability of the EGR valve.
In order to address the above disadvantage, the EGR gas needs to be cut or needs to be substantially reduced during the regeneration of the DPF. As a result, the exhaust gas temperature increasing effect of the EGR or the NOx reducing effect of the EGR cannot be sufficiently achieved. Furthermore, the exhaust gas temperature can be reduced through use of the EGR cooler. However, in such a system that uses the EGR cooler, when the EGR is performed during the DPF regeneration, the EGR gas is cooled equal to or close to the temperature of the coolant of the EGR cooler. Thus, the intake air temperature cannot be increased, and the sufficient exhaust gas temperature increasing effect cannot be achieved.